Current Power Setup
- Motor: Stock 8.5mm Brushed
- ESC: Stock Spektrum AS3X brick
- Prop: Stock 130x70mm w/foam rubber spinner
- Batteries: Eflite 1S 150mAh 45C
Current Radio Gear
- Transmitter: Spektrum DX18 G1 (Prior: DX8)
- Receiver: Stock Spektrum AS3X brick
- Servo, Aileron: 1.8g Linear (SPMSA2005)
- Servo, Elevator & Rudder: 2.3g Linear Long Throw (SPMSA2030L)
- Wingspan: 15.3″ or 388mm
- AUW: 1.7oz or 48g
- CG: 33-34mm aft of wing leading edge
- Flight Time: 6-1/2 to 7 minutes
Manuals, Specs and Instructions
- Plane: Eflite UMX PT17 PDF Manual
Center of Gravity
Factory recommended CG is 31-37mm aft of leading edge. I use the Great Planes CG Machine for setting up center of gravity on my planes but sometimes it does not work for ultra-micros. Some folks use a pair of pencils pushed into styrofoam or wooden dowels attached to a piece of wood. The photo at the top of this page shows how I set CG on ultra-micros, which is just a pair of bamboo chopsticks tightened down in a hand-screw clamp. Width is easily adjusted and it can be propped up at any angle.
Maiden flight was outdoors at a park with just a little breeze. Even with a slightly heavier 150mAh 45C battery (stock is 150mAh 25C), the li’l PT was tail-heavy and required quite a bit of mechanical down elevator trim to fly level. I am happy with the power and flight-time from the 45C battery and it keeps the li’l PT floaty but a 160-180mAh might help more with a little extra nose weight. As usual the AS3X performed great but I would have liked a little more elevator gain. For me the ailerons were extremely anemic, and as you will see below, I did many things trying to get more control authority. I did not see any need for Dual Rates with the li’l PT as it is pretty docile even on high Rates.
I moved the elevator and rudder servo linkages inward on the control horns for more authority. The li’l PT will hover and loop in a 3′ diameter circle with the settings shown below. The elevator and rudder were on the outermost holes from the factory.
Battery Connector Modification
The Eflite 45C batteries are a little longer than the original 25C batteries so there was a slight clearance issue with the battery connector and wires. I modified the connector to minimize stress. I cut some of the plastic from the connector where the wires enter so I could glue them at an angle instead of straight out. I used strong and flexible Goop Automotive glue. This mod keeps the wires from being wedged against the fuselage.
I had a problem with the hook-and-loop fastener not holding the battery securely. I was at the field the first time the battery dangled under the li’l PT and had only a few spare things with me so I tried a rubberband. Not only did it keep the battery from falling out again, it added some cushioning and stiffness to the landing gear. I like doing touch-n-goes and this helps keep the landing gear fairings from denting the fuselage foam.
Aileron Deflection Modification
I wanted to see if I could get more aileron deflection and increase roll rate to match the rest of the li’l PT’s control surfaces. I was able to increase it 2mm by bending the inner aileron control linkage toward the outside of the bellcrank (from green arrows to red, in photo below). Maximum stock aileron deflection on my li’l PT was 4mm with the linkage on the inside and 6mm with the linkage outside, of course with clearance.
To make the center linkage hole in the aileron bellcrank, I heated a small T-pin with a small butane torch and simply pressed it in the plastic. The bellcrank slots are considerably larger than the diameter of the control linkage which adds to the slop. There are also numerous other areas that produce slop resulting in a 2-3mm difference in maximum aileron deflection. I try to address most of these later in this article. You will notice that the li’l PT aileron bellcrank is the same part as the UM T28, UM P51 and probably most of the other ultra-micro planes that use this type aileron setup.
110% TX Aileron Servo Travel
I did extensive testing with linear aileron servo travel on the li’l PT and found that on my particular model I could safely use 110% travel in the TX without locking-up the servo. This might not be the case with all ultra-micro linear servos and should be tested before using. I get away with 125% on a Blade mSR helicopter so all may differ.
The following video is with the above Aileron Deflection Modification and 110% TX aileron travel but with a Dual Aileron Modification (shown below):
Long Throw Servo and Bellcrank Modification
The photo below shows how I cut the black plastic sleeves on the aileron control linkages to allow a broader area to be secured to further reduce flex. Unfortunately the sleeves are a very soft tubing so additional support was minimal.
The photo below shows areas that I glued with CA to reduce linkage slop. I first moved the two linkages outwards so the inside ends were at the outside of the bellcrank slots. The aileron bellcrank mount was glued off center from the factory on my li’l PT so I dug it out and re-glued it.
I replaced the stock 1.8g linear aileron servo with a 2.3g long throw linear servo for extra throw and torque.
The photo below shows a longer aileron bellcrank that I made out of thick lexan and glued to the top of the original. I used servo linkage from a UM T28 Trojan and a 2.8g long throw linear servo from a UMX Sbach 342. The servo is loose in the photo but was glued in the same position as the original.
Dual Aileron Modification
So I decided to do a dual aileron modification to the li’l PT to see if I could get even more authority. I could have used typical ultra-micro biplane hardware but opted for something simpler and lighter. I used 1/2″ transparent tape for attachment/hinges and 0.020″ carbon-fiber rod for linkage. After folding over the corners of the tape, creating a triangle shape, I put a few drops of CA on the ends of the carbon-fiber rods to secure.
Aileron Case Style Servo Modification
I wanted to further test the li’l PT with a standard cased 3g nano servo. I hogged out the foam a little forward of where the stock 1.8g linear servo was originally and glued in the 3g servo with Beacon Hold The Foam. I used the stock servo linkage and just melted a pinhole in the control horn after shortening it. It fit under the brick with room to spare.
Here is a video with the above Aileron Case Style Servo Modification (longer bellcrank, relocated aileron control linkages, and 3g nanolight servo). This yielded 12mm of throw on the li’l PT:
Aileron Control Modifications Conclusion
I increased aileron throw (via bellcrank) first, then tried a longer throw linear servo along with removing most of the linkage slop, then the second set of ailerons in the top wing, and finally the 3g cases servo. I feel the second set of ailerons were not worth doing. I could not get the 3g cased servo centered enough in the fuselage for even weight distribution (side-to-side). My next step before I lost interest was to remove the single servo and all linkages and try two long throw linear servos on the bottom of the lower wing but unfortunately it never came to fruition.
New Purchase Recommendations
If you are interested in the Eflite UMX PT17, I recommend keeping it all stock except modifying the battery connector if running longer batteries and the aileron bellcrank modification perhaps when you open up the fuselage the first time.
Close-ups, Feedback and Reviews
To be more mobile friendly, photos on TazRC are limited in size. I would be happy to post any close-up photos on request. Just leave a comment below. Also, feel free to leave your feedback or personal review on the Eflite UMX PT-17. Registration not required.
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